The present invention relates to a process for coating metallic or ceramic bone replacement parts or dental replacement parts with bioactive substances.
Bioactive substances assist at least partly in the promotion of the growth of the implants on the bone structure in the body. By means of this growth, an implant anchored only purely mechanically in the bones is avoided. As a result, the implant is not subject to any rejection reaction, i.e., it is tissue-compatible, and the connection to the bones is stressable which means that the implant can remain in the body for a sufficient period of time.
According to the present state of knowledge, tricalcium phosphate (TCP) and hydroxylapatite (HA) are particularly suitable as bioactive substances, with HA being preferred.
Implants are already known which partly carry a coating with particles of calcium phosphate, for example from Federal Republic of Germany Patent Specifications Nos. 27 33 394, 28 24 118 and 30 34 086. In Federal Republic of Germany Patent Specification Nos. 33 16 801 and 34 14 992, hydroxylapatite and tricalcium phosphate are mentioned as bioactive materials. However, in both of these references, special measures are mentioned which ensure a firm and secure growth of the implant with the surrounding tissue.
The application of thin layers of ceramic material to metal surfaces is, in principle, known (cf., for example P. Adam, Fortschr. Zahnarztl. Implant., 1, 41-46/1984).
Furthermore, "Sprechsaal," 119, (12), 1131-1134/1986, discloses that all tricalcium phosphates rearrange in water into hydroxylapatite. It is suggested, for the use of calcium diphosphate as an intermediate layer between implant and tissue, to apply a base layer of dicalcium phosphates, which are practically insoluble in water and a surface layer of .beta.-tricalcium or a mixture of .alpha.- and .beta.-tricalcium phosphate. The assumption is made that once implanted, the calcium phosphates on the surface are converted into hydroxylapatite. Corresponding layers can only be produced with difficulty and they are not reproducible and stable.
If it is desired to produce satisfactorily adhering layers on the implant, which can be grown on and can bear loads, then it has proved useful, in addition to selecting appropriate materials, to control the production and sinter conditions of the layer. The porosity of, for example, a sintered apatite material can also be controlled in this way. In the case of the temperatures necessary for sintering, a partial decomposition of the HA takes place, with the loss of its water of constitution, to give tricalcium and tetracalcium phosphate, so that a pure hydroxylapatite is no longer present and the resultant layers are no longer optimal for the growing on of the implant.